I have a time machine and I can use it to go as far back in time as I want.
I know that our atmosphere has changed significantly since the earth was formed so I know I can’t go back 4 billion years. How far back could I go? Would I find anything that I could eat when I get there?
To the best of my limited knowledge of paleoecology, a typical modern person could probably survive even in the Triassic age, without the benefit of hazardous-environment gear.
Aside: Your time machine can project back 4 billion years? I’m impressed – the one I’m working on can only project back about 500 years. Where precisely is your lab? I want to Get There First and examine your system.
I vote for the Cambrian. Then you could go fishing! You’d have to sleep on the rocks, though, or turn the old time machine into a boat, since there wasn’t any land life at the time.
Just be sure to check the maps first. If you live in Hawaii, be sure that island was not still being covered with lava, or you will get a hotfoot. 
About 6,000 years, if the Creationists are right.
It’d be real tough past 200 MYA or so. O2 levels at times were *at sea level * about like living in the upper Andes.
http://uwnews.washington.edu/ni/article.asp?articleID=2205
“Recent evidence suggests that oxygen levels were suppressed worldwide 175 million to 275 million years ago and fell to precipitously low levels compared with today’s atmosphere, low enough to make breathing the air at sea level feel like respiration at high altitude.”
http://uwnews.washington.edu/ni/article.asp?articleID=9592
"Atmospheric oxygen content, about 21 percent today, was a very rich 30 percent in the early Permian period. However, previous carbon-cycle modeling by Robert Berner at Yale University has calculated that atmospheric oxygen began plummeting soon after, reaching about 16 percent at the end of the Permian and bottoming out at less than 12 percent about 10 million years into the Triassic period. …
He calculated that when the oxygen level hit 16 percent, breathing at sea level would have been like trying to breathe at the summit of a 9,200-foot mountain today. By the early Triassic period, sea-level oxygen content of less than 12 percent would have been the same as it is today in the thin air at 17,400 feet, higher than any permanent human habitation. That means even animals at sea level would have been oxygen challenged. "
I’d have to say that’s unlivable in the long term, unless maybe you’re a Sherpa or something. Even so- :dubious:
I am not sure what living in 30% O2 would be like. There also may be CO2 problems earlier.
I’d say limit yourself to the Cenozoic, and you’d be fine. Cretaceous might be OK, if you really want to see Dino’s.
Note that you’d have to limit your area during the Ice Ages. Being under hundreds of feet of ice would be unpleasant. 
In Robert Silverberg’s novel Hawksbill Station there’s a penal colony for political dissidents established sometime in the Paleozoic. I don’t recall exactly when, but there are trilobites for the prisoners to catch and eat, but little or nothing out of the sea. (There’s a colony of female dissidents established a couple of million years away, to prevent unwanted continuing human habitation).
I’m a scuba diver with nitrox certification; 30% is lower than the two normal enriched oxygen levels (32% and 36%). At sea level pressures oxygen toxicity doesn’t begin to be a potential problem until it’s at least over 50%, so that’s not a danger. Barring other toxic gasses in the atmosphere, it’d be refreshing at first, then you probably wouldn’t notice.
Don’t eat the trilobites. One could be an ancestor.
Trilobites are OK, since they left no descendents of any kind. You would probably want to avoid the slime eels, however.
I like time-travel sci-fi.
Is the book as good as it sounds?
To be more precise, 5766 years, 2 months, and 18 days. But be careful: Some of those last “days” don’t last precisely 23 hours 56 minutes & change.
I thought trilobites *were * sea creatures.
My expression was ambiguous. I meant that little or nothing was living out of the sea. (So there wasn’t much to catch)
Ah, I see. Thanks for the explanation.
I don’t know what CO2 levels were like back then, but I wouln’t think that longterm survival would be possible much abouve 0.1%. 0.5% is allowable for a normal person in an eight hour work day, but breathing it 24/7 I’d say it has to be even lower.
Way before CO[sub]2[/sub] levels got anywhere near 0.1% the greenhouse effect would have rendered the Earth too hot to inhabit.
When I went to camp, they separated us from the females by a lake. How times have changed…
Personally, I enjoyed it. It’s been about 25 years since I read it, but if I came across a copy again, I’d reread it.
If you like it, you might also try his Up the Line, about travel to ancient Byzantium.